These lines should go out in the bsdinstall auto file, after commenting out bsdinstall autopart and bsdinstall mount section. Remember, this would wipe out the entire disk contents without any warning, so use it at your own risk. I am also assuming that if you have multiple disks connected, the first disk is what you want to install your OS on. Otherwise, you might have to modify the script accordingly.

Get the disk layout, and the first disk connected to the system# disk_layout=`sysctl -n kern.disks`
# first_disk=${disk_layout%% *}

Create a boot partition to hold the loader, size of 512K. Give it a GPT label of gpboot, which will show up in /dev/gpt when the device is discovered:# gpart add -t freebsd-boot -l gpboot -b 40 -s 512K $first_disk

Create a partition for /. It should start at the 1M boundary for proper sector alignment on 4K sector drives or SSDs. This is compatible with GPT drive layout for many other systems. Give it a GPT label of gprootfs.# gpart add -t freebsd-ufs -l gprootfs -a 1M $first_disk